Recently, LED luminaires which use LED light sources, particularly white LED light sources, are widely employed.
An LED light source is highly directional. In order to alleviate glare, an LED luminaire having an LED light source is provided with a semi-transparent light diffusion cover (a light diffusion cover for an LED luminaire) which surrounds the LED light source and which has a light diffusion function for diffusing light emitted from the LED light source.
Conventionally known light diffusion covers for LED luminaires include a light diffusion cover for an LED luminaire in which transparent inorganic particles (such as calcium carbonate particles and barium sulfate particles) or transparent resin particles (such as crosslinked acrylic-based resin particles and crosslinked acrylic-styrene copolymer particles) are blended in a transparent thermoplastic resin (such as an acrylic-based resin or polycarbonate).
In a spectral irradiance distribution of a typical LED light source used for an LED luminaire, peaks appear at around 450 nm and around 550 nm. Since light having the shorter peak wavelength around 450 nm is glaring to the human eyes, a light diffusion cover for an LED luminaire which effectively diffuses light around 450 nm is practical for alleviation of glare.
A wavelength region between 380 nm to 500 nm is known as a blue light region. Light having the shorter peak wavelength around 450 nm is in this region and is called blue light. Recent studies warn that blue light is harmful to the human eyes. Hence, a light diffusion material for diffusing light in the blue light region, which can reduce rectilinear light in the blue light region, is assumed to be useful for eye protection.
Therefore, it will be effective for alleviation of glare and eye protection if a light diffuser (such as a light diffusion cover having a wavelength-dependent light diffusion property) in which the light diffusion property around 450 nm is prominently greater than the light diffusion property at the other wavelengths becomes available.
Nevertheless, no one has been working on a technology for imparting wavelength selectivity in light diffusivity to a light diffuser (such as a light diffusion cover) such that the light diffusivity around 450 nm is prominently greater than the light diffusivity at the other wavelengths.
In the meantime, there has been proposed an optical sheet for a liquid crystal display such as a PC monitor and a television, having a reduced wavelength dependence in order to mitigate a color change due to a viewing angle (see Patent Literature 1).